Spiral wound antenna with controlled spacing for impedance matching



Aug. 27, 1963 w, H. FQLEY 3,102,268

SPIRAL WOUND ANTENNA WITH CONTROLLED SPACING F 0R IMPEDANCE MATCHING Filed April 1l, 1960 2 Sheets-Sheet 1 INVENTOR. f7. .ZY-o [ey e I u" In. Il. .l Il x f Ill llllllllllllllll /uw ma, ATT-ORN EYS iam maw

FOLEY SPIRAL WOUND ANTENNA WITH CONTROLLED Aug. 27, 1963 w. H.

SPACING FOR IMPEDANCE MATCHING 2 Sheets-Sheet 2 F'iled April l1, 1960 INVENTOR. (/V//J'a/W F0 ley BY usw PQM@ A T TORNEY s.

` V,snm/SL woUND "ANTENNA WITH skin or surface of the conductor.

CONTROLLED SPACING FOR IMPEDAN CE MATCHING i Williamfl-I. Foley, New Boston, Mass.,` assignor, `by

` mesne assignments, toBrunswick-Union Inc., a'corpo-` ration of Delaware f i This. invention relates toa radiationqsystem andmore of my application Serial No. 580,1l6,-filedfApril 23, 1956, nowabandoned` 1' f v Modernfdevelopments of communication makeit necessary to have a means of `transmitting wide bandsof frequencies. `In some instances the antennas whichhavebeen utilized forthis purpose have 'takenthe form oft-acylin- .drifcal tube which may or may not be telescoping and which is generally made outof a range of *materialspincluding steel, copper, tin,and aluminum. Itisrwell knownth-at radio frequency energy is conducted .on the Because of this physical` phenomenon somediiculty 'has been experienced with certain `materials `which have oxidized orotherwise been affected' in corrosiveY atmospheres,

`to .inhibit the oxidation effects, but :in themselvesthey only offer `a solution to a portion of the antenna problem.

. -In communications, preoipitationstatic .;is ajserious s problem;` .Precipitation static technically is ,a jcorona dischargewhichis an electrostatic impulser that occurs more or less regularly with time and frequently is heard `in a receiver assenti-musical or screaming in character., These i discharges effectively block radio communications for the duration thereof. It has been found that this electrical phenomenon; is not associated alone with thunderstorms but occurswith `all types of precipitation including snow,

-raiin; ,and dust and is` not limited to moving antennas.v `Bare antennas, therefore, may very well be one of the' rst elements to start dischargingbecause of the small radius i of curvature compared to the other portions of the sysy tem; It is, `of course,l well vknown .that the potential gradient required to produce corona on a conductor depends upon the'radiusthereof, the surface electric field being inversely proportional to the radius. vFor practical reasons, however, a .limit'soon Uintervenes before the conductor-may be unduly f enlarged toreduee the effect.

This problem suggests .that if coronacurrentiin an ani tenna can be completely suppressed by covering the antennawith an insulatingjlayer,` the antenna would .be

satisfactory,` Tests have indicated that certain synthetic` materials meet the physical ,and electrical qualifications for insulating against this.phenomenon,swhich'characteristics `resolve themselves into avery highl electric resistance, highdi-electric strength, a low radio frequency loss, a minimum deterioration dueto ultra-violet radiation .and an ability to withstand the normal operating temperatures encountered. Y f i 1,

Itis also' lhighly desirable in an lantenna to have a rather effective impedance matchV between the antenna and the inputl circuit. yOne of thejcommonly used input im-` pedances is 52"v ohms. To achieve .this impedance, there "arecertain physical factors involved in antenna design which vcan `be related, suehas the length-diameter .ratio and the. electrical length, which willietfect this proper impedance match. These various` design considerations `are summarized by, Brown and `Woodward inthe Proceedings of the IRE, April 1945, pages 257-262, and` in-l dicate that in order to obtain a pure resistive impedance termination of 52 ohms, the length-diameter ratio is a .critical one.v In certain frequency ranges this lengtht t Coatings ofV various Atypes have, of course, been used `and suggested United-Safes Patent o `particularly to an antenna andis a continuation-impart er.' l l ICC 2 diameter ratio may produce an antenna of rather large diameter proportions which introduces a weight and size problem that cannot readily be solved with existing structures. V i

rlhe antenna cons tnuction of the` instant invention takes into account the aforementioned lproblems, and produces ankantenna whose length-diameter ratio may be readily varied without unduly increasing the weight thereof and whose impedance for a chosen length-may be varied. The resulting antenna is. therefore one which vrenders vconsistent radiation `results and maintains f a" constant impedance termination in a variety of operating environments. Somefattempts at producing insulated antennas);

are known, one of the prior art structures beingl exemplilied in the `Closson Patent No.42,373,660 which embodiesl a wire embedded ina synthetic material.` This antenna, while satisfactory forthe reception of radio waves in a high density lieldzstrength, isnot satisfactoryfor `twowaycommunicationswhere high efficiency is vnecessary as the impedance characteristics are particularly pooras will be exemplied by the cited Brown and Woodward article.

. One of the main objects, therefore, of this invention is to produce an antenna vstructure which will vbe light in weight.`

A principal object of the invention is Ato produce an antenna structure-whichV can vbe readily manufactured to l provide a pure resistiveterrnination to a co-axial feed line.

Another object of the invention is to produce a structure which is susceptible ofhaving itsytermination impedance varied once its length-diameter ratio has been determined to obtain van efficient impedance match.

" J A further object of thisdnvention is to produce an anla sectional viewl of tahetopV portion of one Y form of my antenna;

FIGURE 4` is an elevational view, of a portion of my antenna showing one manner of construction;

FIGURES is a cross sectional view of a modified form; f

FIGURE 6 is a `cross sectional View of still another modified form;

FIGURE 7 is a detached sectional view of my antenna in coaxial form;l f f FIGURE (il isa ypartial elevationalvview of a portion of my` antenna without the-outer sheath and` having a loading coil which is illustrated in section;.V t

FIGURE 9 is a longitudinal sectional view of a further modified form of the antenna.- f i -In proceeding with this invention and by lway-of exy ample of one method of manufacturing the antenna, I

take insulatingmaterial which maybe impregnated with a suitable insulating resin and form this material into a suitable cross sectional shape. .'llhe resulting structure n is then placed'in a braiding machine or Winder and an i electrical conductor may be either braided or helically wound thereon withr suitable pigtails left at either end for later fabrication. Upon the conductor another layer of insulating material is formed, this material being also suitably impregnated `with a synthetic resin, and the entire structure is then placed inan oven for curing `of the resin in accordance with techniques developed for the t 1 yresin in use so'as to establish a homogeneous structure.Y

Patented' Aug; 27, 1963 Referring to the drawings, the antenna shown therein consists of a core `10, which is preferably composed of a material impregnated with a suitable resinous insulating material such as a phenolic. The material utilized may, of course, take many forms illustrative of which would be a resinous sheet with reinforcing fibers, a woven material, laminates, etc. The core may assume any shape, such as elliptical, cylindrical, or polygonical, as shown in FIG- URES 5 and 6, and may be hollow or solid. The core may be formed of a strip or sheet of material wound upon a rem-ovable mandrel M or solid insulation of any polygonieal cross section such as shown in FIGURE 6 coated with an insulating resin. This core has formed .thereon a conductor 11 shown as composed of braided single strands of wire which are bare. The conductor 11 may, however, be varied by winding bare wire strands over the core in successive opposing helices if desired, which will yield a structure that from outward appearance will be very similar to a braid. It will be apparent, therefore, that at each of the crossings of the wires a ratherI good electrical contact is maintained, the braiding or winding in eifect forming a conductor of appropriate diameter in accordance with the criteria discussed above. The outer sheath 12 consists of a material similar to the core and is likewise impregnated with Ia suitable resin. The sheath may be formed over the conductor so as to completely cover the same (see FIGURE 4), lor a preformed covering may be utilized. Although in the drawings, by way of example, I have shown the sheath 12 as wound or laid in the same direction as core 10, it will be understood that under certain conditions, in order to impart additional strength to a structure that is formed as illustrated, the core and sheath may be wound or laid in opposing helices. The conductor may be further varied by using a single helical winding 11" (-see FIGURE 9).

Tapering of the antenna structure to the general outline of FIGURE 1 may be readily effected by utilizing known techniques such as used inthe fishing rod industry. It Will be apparent that I may change the spacing of the individual group of wires of the helices or braids, and in experiments that have been conducted, I have found that the impedance of the antenna may be nicely varied over a rather narrow range by varying this spacing without changing the diameter of the core 10. After the structure is completed, it is placed in an oven of appropriate temperature, for the resin in use, to cure the resin. Y The curing makes the antenna a homogeneous mass. As a result thereof, a perfect bond is maintained between the inner core and the outer sheath, which effectively embeds the conductor 11 within the resin and prevents the access of `air to the braided or wound conductor. Also by utilizing this construction, the antenna may be readily flexed into `slightly more than a 90 degree bend without undue distortion of the cross sectional shape thereof.

A suitable mounting ferrule 14 may be placed over one end of the antenna, which is made in accordance with the procedure suggested above. This ferrule is provided with means for attachment of the conductor l11 thereto, such as a bore 18, which is adapted to receive a pigtail 11a that may be soldered therein. The lower end of the ferrule has an end 1S suitably threaded for attachment to various antenna mounts represented at 20 which have a coaxial feed line 21, while the upper end of the ferrule has a supporting sleeve 1 9. The other or tip end of the antenna may be left plain .or finished with -a ball 16l and sleeve 17 secured thereto. The method of securing may vary; for instance, if the parts are metal, the pigtail 11b may be soldered to the lball 16.

The above procedure results in an antenna of the vertical whip type that if made in conjunction with certain techniques will be immediately ready for use. The techniques involve first determining the length of a quarter wavelength at the operating frequency, or at least nearly so taking into account end-effects and the like, and then calculating the proper length-diameter ratio to achieve a characteristic impedance which will match the feed-line to the antenna. Co-axial feed lines are generally employed for this type of radiator and for whip antennas will have a characteristic impedance somewhat in the vicinity of 52 ohms. It is, of course, well known that the impedance of a whip antenna can be controlled to a degree by the length-diameter ratio as pointed out above. Still such a design consideration will lead one to an approximate impedance termination but does not give one exact impedance. To provide this exact impedance, as suggested above, the spacing between the individual wires can be varied until by experiment a perfect termination has been secured widening the spacing lowering the impedance and Vice versa. Such gives the antenna feed line practically zero standing waves and yields an antenna of utmost eiciency in operation. Accordingly, in the physical structure actually produced, an improved result is obtained by varying the size of the spacing between the individual' strands to provide the proper impedance, a result that cannot be successfully accomplished by trimming the length for the current loop must be kept in optimum position of the antenna.

I have also shown in FIGURE 7 of the drawings an antenna which is particularly adapted for broad band operation and which consists of ya second conductor element located in spaced relation to the rst element at one end of the antenna. This produces a coaxial antenna, or a hypodermie antenna or sleeve type antenna, las it is sometimes called, and which lends itself to a broad band effect of operation. This antenna comprises a core 20 upon which there is superimposed a conductor 21 which terminates at either end thereof in pigtails 21a, 2lb. Pigtail 2lb is secured to a tip 26, while pigtail 21a is secured to the center coaxial contact pin 23 of connector 24.

o Surrounding the conductor 21 is an insulating covering 22 which extends the full length of the antenna. Braided or otherwise for-med over this covering 22 is another conductor 30 which in effect forms the outer sheath of what might be termed a coaxial cable that extends from the connector 24 to point X. Spaced slightly from point X and extending into the connector 24, the conductor 30 is covered with insulation 31. Over this insulation 31 a third conductor 32 is braided from point X toward the connector 24 a distance in the order of one-quarter wave length for the mean frequency of operation, it being understood that the antenna above point X is usually but not necessarily the same order of length of the antenna below point X. It will be noted that by the arrangement of the insulation 31, the conductors 30 and 32 join at point X, which juncture may be enhanced by soldering or any other bonding technique. Covering the conductor 32 is a layer of insulation 33' which is bonded to insulation 22 as at 34, 34. Any suitable connector 24 may be utilized, the illustrated form being shown merely by way of example to include suitable conductor shield gripping means 25 in the form of a wedge actuated by nut 26. It should further be understood that the dimensions of the antenna structure are not to scale, for, as is well known in the art, the diameter of conductors 30 and 32 must be chosen to provide an impedance in the vicinity of 70 ohms Iand also that the insulating coverings are bonded together as described in connection with the lirst embodiment.

IIn FIGURE 8 I have shown a further adaptation of any antenna construction in which a loading coil may be inserted yat `any appropriate point therealong, preferably near the center thereof, to raise the current lobe in the lower section. Commercial loading coils Iare generally bulky and add lgreatly Ito the weight and moment of the antenna, 'which'results in a tendency to swing violently, a result eliminated with -my construction. Briey, lche core 10 has formed thereover a conductor that is in two sections 11a and 11b', the sections being joined together by -a coil 19 wound over the core 10. As in the previous embodiments, the entire structure is then covered with Kan insulating material suitably irnpregnatedand then cured to yield -a homogeneous antenna with loading features and extremely small diameter.`

It will 'thus be apparent that I have produced an antenna construction which is readily adaptable :for manuacturing variations and in which the RF losses are considerably less than the skin losses exhibited with other conductors.. The factor :of less loss comes about through the phenomenon of providing a plurality of small conductors, which in eiect produce -a larger `surface area for the conduction of nadio frequency energy. Tlhus, for any given diameter of a braided structure vs. ia tubular structure there will be present more sur-face tarea for the energy to be conducted therealong. ln addition to providing a large conductive area, the conductive part of the -antenna may be nicely varied to provide lan impedance match to -a co-axial feed 4line which is accomplished by varying the spacing between the wires and the antenna structure after using the primary design technique of determining the optimum `length-diameter ratio.

lclaim: y

1. A radio frequency antenna which is adapted to be connected to Ia radio frequency transmission line having a predetermined characteristic impedance, said antenna comprising: a icore of dielectric material having rst and second bare wire strands encircling said core in helical tiashion t form "an antenna having a Ilength-diameter ratio which is chosen to iachieve said predetermined characteristic impedance' approximately, said iirst and second strands lbeing wound about said core in opposite directionsto cross eachother and make rcontact at the crossing points to form tinite spaces between said strands, said antenna being improved by selecting for each oi the ywound strands ya particular pitch angle within a mange of pitch angles that effect the resistance component of the impedance of said antenna, said resistance component changing from a larger value =for the smallest pitch angle in said range to a smaller value for the nal-gest pitch angle in said range, whereby said predetermined characteristic impedance is achieved precisely by the selection of said particular pitch angle. f

6 2. In combination a transmission line, lan antenna having la length of approximately one quarter wave length at the mean frequency to be radiated therefrom and of Ia predetermined characteristic impedance, and a transmission Iline `connector means for connecting said antenna to said transmission line, said antenna comprising a supporting jcore coated with resinous material and )a plurality of individual spaced bare wire strands encircling said core in helical ifashion, each strand crossing a plurality of other individual spaced bare wire strands that encircle said core in opposite helical fashion, each of said strands being in cont-act with each other at the ycrossings thereof and leaving iinite spaces between adjacent strands, the length- `diameter ratio of said strands being 'arranged to produce said predetermined characteristic impedance approximately, said finite spaces having a particular width mat is selected from a band of widths that Iaec-t the resistance component of the impedance of said antenna, said resistance `component ychanging from a 'larger value for the narrowest width in said band to a smaller value for ythe widest width in said band, whereby said predetermined :characteristic impedance is achieved precisely by the selecticn of said particular width.

Reiterer-aces Cited in the iile of this patent Y UNITED STATES PATENTS OTHER REFERENCES Antennas, by I. D. Kraus, McGraw-Hill Book Co., 1950, page 244 relied on. 

1. A RADIO FREQUENCY ANTENNA WHICH IS ADAPTED TO BE CONNECTED TO A RADIO FREQUENCY TRANSMISSION LINE HAVING A PREDETERMINED CHARACTERISTIC IMPEDANCE, SAID ANTENNA COMPRISING: A CORE OF DIELECTRIC MATERIAL HAVING FIRST AND SECOND BARE WIRE STRANDS ENCIRCLING SAID CORE IN HELICAL FASHION TO FORM AN ANTENNA HAVING A LENGTH-DIAMETER RATIO WHICH IS CHOSEN TO ACHIEVE SAID PREDETERMINED CHARACTERISTIC IMPEDANCE APPROXIMATELY, SAID FIRST AND SECOND STRANDS BEING WOUND ABOUT SAID CORE IN OPPOSITE DIRECTIONS TO CROSS EACH OTHER AND MAKE CONTACT AT THE CROSSING POINTS TO FORM FINITE SPACES BETWEEN SAID STRANDS, SAID ANTENNA BEING IMPROVED BY SELECTING FOR EACH OF THE WOUND STRANDS A PARTICULAR PITCH ANGLE WITHIN A RANGE OF PITCH ANGLES THAT EFFECT THE RESISTANCE COMPONENT OF THE IMPEDANCE OF SAID ANTENNA, SAID RESISTANCE COMPONENT CHANGING FROM A LARGER VALUE FOR THE SMALLEST PITCH ANGLE IN SAID RANGE TO A SMALLER VALUE FOR THE LARGEST PITCH ANGLE IN SAID RANGE, WHEREBY SAID PREDETERMINED CHARACTERISTIC 